Decoding Pure Rotational Molecular Spectra for Asymmetric Molecules

Abstract

In this paper we demonstrate how asymmetric molecular rotational spectra may be introduced to students both "pictorially" and with simple formulae. It is shown that the interpretation of such spectra relies heavily upon pattern recognition. The presentation of some common spectral patterns in near-prolate asymmetric rotational spectra provides a means by which spectral assignment, and approximate rotational constant determination, may be usefully explored in the physics and chemistry classrooms. To aid in this endeavor we have created a supporting, free, web page and mobile web page.

Microwave spectrum, structure and dipole moment of 3-fluorophenylacetylene (3FPA)

“…Compared to the results of ab initio calculations, these values are much closer to the constants from MP2/6-311++G (2d,2p) basis set than from MP2/6-31+G(d,p). Ray's asymmetry parameter [22], ĸ = (2B−A−C)/(A−C), is −0.75625, which is close to so-called near-prolate asymmetric top (κ ≈ −1) [23]. Four quartic centrifugal distortion constants of 3FPA were fitted from the Watson A-reduction.…”

Structure of fenchone by broadband rotational spectroscopy

“…25 Once these lines were identified (see a-, b-, and c-type transitions (Table S1 of the supplementary material) were fit using the Watson Hamiltonian in the A reduction and I r representation 26 and Pickett's program 27 to give the rotational and quartic centrifugal distortion constants in Table I. The centrifugal distortion constants are quite small, which is an indication of the rigidity of fenchone.…”

Automated, Context-Free Assignment of Asymmetric Rotor Microwave Spectra

“…where ν 1 is the frequency of the lowest a-type transition in this series, and ν 2 is the frequency of the second lowest transition (see for example the analytical approximations described in Cooke et al [1]). Although we find scaffolds in most cases with K a values of 0, 1, or 2, we try assignments up to K a = 4.…”

“…Today, such assignments are typically performed using a combination of quantum chemical simulation, which can in most cases predict rotational constants to within a few percent, and often laborious inspection of the observed spectrum. Assignment of complex spectra remains very much an art, and veteran spectroscopists employ diverse tricks as well as deep intuition to find requisite patterns amid congested spectra [1].…”

“…Today, such assignments are typically performed using a combination of quantum chemical simulation, which can in most cases predict rotational constants to within a few percent, and often laborious inspection of the observed spectrum. Assignment of complex spectra remains very much an art, and veteran spectroscopists employ diverse tricks as well as deep intuition to find requisite patterns amid congested spectra [1].Robust automatic fitting of rotational spectra or rovibronic spectra has been a longstanding goal of the spectroscopy community. Attempts at fully automated algorithms include genetic algorithms [2], broad searches combined with quantum chemical calculation [3,4], assignment via nonlinear spectroscopy [5,6], and artificial neural networks [7].…”